In recent years, as the digitization of electronic devices proceeds, it is demanded that impedance (equivalent series resistance) in a high frequency region of capacitors used for electronic devices should be reduced. Conventionally, to meet the demand, so-called functional capacitors (hereinafter abbreviated to capacitors) have been used in which an oxide film of a valve metal such as aluminum, tantalum or niobium is used as a dielectric.
As shown in Patent Document 1, the structures of these capacitors commonly include an anode composed of a porous material of a valve metal, a dielectric layer formed by oxidation of the surface of the anode, a conductive solid electrolyte layer, and a cathode in which a carbon layer, a silver layer and the like are laminated. As the solid electrolyte layer, a conductive film containing a conductive polymer may be used.
As methods for forming the conductive film containing a conductive polymer, electrolytic polymerization methods in which a conductive layer composed of manganese oxide is formed in advance on the surface of a porous material of a valve metal and this is then used as an electrode and energized to carry out polymerization (see Patent Document 2) and chemical oxidation polymerization methods in which a precursor monomer which forms a conductive polymer is polymerized using an oxidizing agent are widely known (see Patent Document 3).
As a method for forming a conductive film other than the electrolytic polymerization methods and the chemical oxidation polymerization methods, for example, Patent Document 4 proposes a method, that is, aniline is subjected to chemical oxidation polymerization in the presence of a polymer anion having a sulfo group, a carboxy group or the like to prepare a water soluble polyaniline and an aqueous solution of the polyaniline is applied and dried to form a coating film. This method is believed that a conductive film with high conductivity can be easily formed.
The present inventors have already disclosed a capacitor including an anode composed of a porous material of a valve metal, a dielectric layer formed by oxidation of the surface of the anode, and a solid electrolyte formed on the dielectric, wherein the solid electrolyte is formed from a composition including a conductive polymer to which a nitrogen-containing aromatic compound is added (PEDOT/PSS polymer dispersed liquid) as an essential component.
The features of the composition forming the solid electrolyte are that functional capacitors such as an aluminum electrolytic capacitor and a tantalum capacitor can be easily produced (see Patent Document 5).
Meanwhile, an attempt to use conductive polymer dispersions as a solid electrolyte of a capacitor has been already carried out for more than 10 years. As additives of the conductive polymer dispersions as above, a conductive complex of a cationized conductive polymer and a polymer anion (see Patent Documents 1 to 5) and a hydroxy compound having an amino group and one or more hydroxy groups (Patent Documents 6 to 14) are disclosed.